Csir Qn Papers

Prepared by V. Aditya vardhan adichemadi(at)gmail.com WARANGAL

V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

1

This is updated on 22nd Nov, 2009 and going to be updated frequently. For recent updatesand other parts, visit

http://www.adichemadi.com

BIOINORGANIC CHEMISTRYNote: Questions solved from csir and gate exams

41) Complexes of which of the following metals are used in the treatment of rheumatoid ar-thritis:1. Gold2. Ruthenium3. Iron4. Copper

Explanation* Gold salts (containing Au(I)) like Auranofin, Sodium aurothiomalate, aurothioglucose areused to treat rheumatoid arthritis.

Additional information

Some applications of metals in medicine* cis platin and budotitane are used in treatment of cancer.* Iron in the form of ferrous sulfate, ferrous gluconate are used in treatment of iron deficiencyanemia.* Li+, in the form of Li2CO3, is used in the treatment of depression, hypertension.* Sb(III) salts are used in eczema (inflammatory condition of skin).* Bi(III) salts (as Bismuth subsalicylate ) are used in gastric ulcer.* BaSO4 is used as contrast agent in radiography.* Gd3+ is used as contrast agent in NMR.* 99mTc (in Cardiolyte) is used in radio diagnostics. 99mTc is a metastable isotope of Technetium,an artificially made element. It’s half life is 6hrs only and emits gamma rays.* Silver sulfadiazine is used to treat and prevent bacterial or fungal infections of the skin.* Selenium sulfide used to treat seborrheic dermatitis and Tinea versicolor.* MoS4

2- (tetrathiomolybdate) is used as “anti copper agent” in Wilson’s disease (excess ofcopper accumulation in liver - a genetic disorder). It is also used as an antitumor agent.

Inorganic elements and their biological functionsBulk metals - Na, K, Mg, CaTrace metals - Zn, Fe, Co, Ni, Cu, Mo, V - metals with low conc. are used for biocatalysis.

* Na+,K+: As electrolytes, maintain the concentration gradient (osmotic balance).Helps in active and passive transport.Charge carriers.

* Mg2+: Present in chlorophyll.In energy production (ATP --->ADP);Activation of enzymes.Information carrier;Present in endo and exo skeletons.

* Ca2+: Charge carrier.In muscle and nerve functions - cell signalling. It acts as second messenger and sen-


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

2

tinel at synapse.Present in teeth as Ca5(PO4)3(OH) (hydroxylapatite), and CaCO3,Present in endo and exo skeletons;In activation of enzymes;In blood coagulation.

* VIV/V, MoIV/VI, WIV/VI, MnII/III/IV, FeII/III, NiI/II/III, CuI/II: electron transfer* Fe and Cu: Transport and storage of dioxygen.

Fe3O4 is used to store iron, and, as it is magnetic, is used by ‘magnetotactic’ bacteriato sense the direction of the Earth’s magnetic field.* Co: Cobalamine, e.g. Vitamin-B12* Mn: In photosynthesis, generation of dioxygen by splitting water. Mn is part of OEC(Oxygen Evolving Complex) in PS II.* Mo, Fe & V: Conversion of N2 to ammonia (nitrogen fixation).* Zn2+: Enzymes, zincfinger proteins (genetic transcription), stabilization of proteins.* Si(IV) Bones.* P5+: Hydroxylapatite, ATP, cell membrane, DNA.* Se(II): Selenocysteine* F-: As fluorapatite (Ca5(PO4)3F) in teeth.* Cl-: Most important free anion, besides HCO3

-

* I-: functioning of hormones of the thyroid, in radiation therapy.* Ni2+: Hydrogenase and hydrolases (urease).

Bioactivity and accompanying metals* Electron carriers Fe : cytochrome, iron-sulfur protein;

Cu : blue copper protein.

* Metal storage compound Fe : ferritin (related is “transferrin” which transport iron)Zn : metallothionein.

* Oxygen transportation agent Fe: hemoglobin (related is “myoglobin” which stores O2)Cu: hemocyanin.

* Photosynthesis Mg: chlorophyll in PSIMn: part of OEC in PSII

* Hydrolase Zn: carboxypeptidase.Mg: aminopeptidase.

* Oxidoreductase Fe: oxygenase, hydrogenase.Fe, Mo: nitrogenase.

* Isomerase Fe: aconitase.Co: vitamin B12 coenzyme.

% of Elements in Earth Crust and in Human Body


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

3

Element % Element %O 47 O 63Si 28 C 25.5Al 7.9 H 9.5Fe 4.5 N 1.4Ca 3.5 Ca 0.31Na 2.5 P 0.22K 2.5 K 0.08

Mg 2.2 S 0.06

Earth crust Human body

More stuff - with brief explanation* Superoxide dismutase, present in the cytosol, is a Cu-Zn containing enzyme catalyzing thedisproportionation of O2

- (superoxide) to O22- (peroxide) and H2O.

* Hemocyanins are respiratory metalloproteins containing two copper atoms that reversiblybind a single oxygen molecule (O2).* In terms of abundance in the human body, zinc is the most important trace element after iron.Zinc is present in

i) carbonic anhydrase, an enzyme, which converts HCO3- to CO2.

ii) zinc finger proteins which recognize specific DNA sequences and are involved in genefunction.

iii) Liver Alcohol DeHydrogenase (LADH), which facilitate the inter conversion betweenalcohols and aldehydes (or ketones).* Normal nitrogenase enzymes contain Mo and Fe, but less common forms with vanadium arealso known. Nitrogenase enzyme catalyses conversion of N2 to ammonia.* Vitamin B-12 contains Co(III). There is a corrin ring system in it. (Can you mention thedifference between heme and corrin ring systems?)* Among the metals present in human body, the most abundant is Calcium.* Be, Cd, Hg, Tl and Pb are toxic elements. These elements have strong complexing ability andan especially strong affinity for sulfur. They may displace essential elements such as Ca and Fe,and may also disrupt protein structure by breaking S-S bridges. Once attached to suitable ligandsthey are hard to displace.

Chelation therapy is used in treatment for heavy metal poisoning. It uses chelating ligandslike EDTA that bind very strongly with toxic elements in complexed form and remove them.

Additional questions:41.1) Give the structure, hybridization and magnetic moment of cis-platin? Write its action

on cancer cells.

Ans:- cis-Diamminedichloridoplatinum(II)

Note: chlorido is very recent IUPAC usage instead of chloro.* Pt (Z=78). Belongs to Nickel group. with e.c - [Xe] 4f14 5d8 6s2

* For Pt2+ -- [Xe] 4f14 5d8 .* Hybridization of Pt2+ is dsp2 . It is a square planar and low spin complex with zero magneticmoment. The crystal field splitting of square planar complexes is shown below.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

4

Action: Upon administration to the cancer patient, the chloride ligands are displaced by waterand thus aqua platinum complexes are formed in cells, which bind and cause crosslinking ofDNA---- ultimately triggers apoptosis - programmed cell death.

41.2) Why are d-metals such as Mn, Fe, Co, and Cu are present in redox enzymes in prefer-ence to Zn, Ga, and Ca?

Ans:- Mn, Fe, Co, Cu occur naturally in redox enzymes because they can have at least two stableoxidation states. Redox reactions involve the cyclic oxidation and reduction of the metal ion.The other metals i.e., Zn, Ga, Ca, have only one stable oxidation state, and hence cannot beoxidized or reduced at physiological potentials.

However Zn and Ca are also present in biological systems to carry out other functions.

41.3) What are ferritin, apoferritin and transferrin?Ans:- Ferritin is a globular protein complex consisting of 24 protein subunits and is the main

intracellular iron storage protein in both prokaryotes and eukaryotes. It has the shape of ahollow sphere. Inside the sphere, iron is stored in the Fe(III) oxidation state. It is incorpo-rated in the mineral ferrihydrite, [FeO(OH)]8[FeO(H2PO4)], which is attached to the innerwall of the sphere. Whenever required by the body, iron is reduced and released as hydratedFe(II).

Ferritin that is not combined with iron is called apoferritin.Transferrin is a glycoprotein present in blood plasma that binds Fe(III) very tightly but

reversibly. Affinity to iron decreases with decrease in pH. It helps in transport of iron.

41.4) What is the function of ‘Mn’ in photosynthesis?Ans:- ‘Mn’ is present in Oxygen Evolving Complex (OEC) of photosystem-II. It helps in genera-

tion of dioxygen by oxidising water molecule.The OEC is a cluster compound containing fourMn ions (probably two in Mn(II) and Mn(IV) states). These help in electron tranfer reactions.

(Glu)O N(His)

(Glu)O(His)N

OOH

O

Mn

Mn

O

Ca

O

O

Mn

Asp

MnOH2

OH2

O(Glu)

OH2Asp

OH2

OH2

Oxygen Evolving Complex (OEC)


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

5

41.5) What is the active site in carboxy peptidase? Mention its role?Ans:- The active site in carboxy peptidase contains tetrahedrally coordinated Zn2+. It is coordi-

nated to two histidine residues, one glutamate residue and one water molecule.

Zn2+

OH2

His(N)

His(N)O(Glu)

Funtion: It is a hydrolase enzyme. It removes C-terminal aminoacid from a protein. Thisprocess is repeated until all the amino acids are removed from C-end. Thus this enzyme helpsin degradation of peptides in biological systems.Mechanism: i) Zn2+ activates water molecule for nucleophilic attack ii) H2O is polarized by anucleophile (base) iii) polarization of carbonyl bond which is to be cleaved.

Do you know the Edman degradation (in the laboratory) is a method for removing the N -terminal amino acid? If you don’t, just remember.

41.6) What is the active site in carboxy anhydrase? Mention its role?Ans:- The active site in carboxy anhydrase (carbonic anhydrase) contains tetrahedrally coordi-

nated Zn2+. It is coordinated to three histidine residues and one water molecule.

Zn2+

OH2

His(N)

His(N)(N)His

Function: It catalyzes the conversion of carbondioxide to bicarbonate.

CO2 + H2O H+ + HCO3

-Carbonic anhydrase

Mechanism:Step 1: Deprotonation of coordinated water molecule. This is crucial step. The zinc bound

water is more acidic than free H2O and loses proton easily.Step 2: Thus formed zinc bound hydroxyl group carries out nucleophilic attack on CO2 to

get (His)3Zn-OCO2H complex. (IT IS A NUCLEOPHILIC ADDITION ON CO2)Step 3: Displacement of -OCO2H by water molecule.

(His)3Zn2+

-OH2

(His)3Zn+-OCO2H

(His)3Zn+-OH

CO2

H2O

-HCO3-

H+

41.6) What is the active site in Liver Alcohol Dehydrogenase (LADH)? Mention its role?Ans:- The active site in carboxy anhydrase (carbonic anhydrase) contains tetrahedrally coordi-

nated Zn2+. It is coordinated to two cysteine residues, one histidine residue and one watermolecule.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

6

Zn2+

OH2

Cys(S)

Cys(S)(N)His

Function: It catalyses the oxidation of primary and secondary alcohols to the correspondingaldehydes or ketones by the transfer of a hydride anion to NAD+ with release of a proton.

R1

R

OH

H+ NAD

+ LADH

R1

R

OH

++ NADH +

41.7) Studies of Zn(II)-containing proteins often make use of Co(II)-for-Zn(II) substitution.Which statement is correct?a) Tetrahedral coordination is one of several environments observed for both Co2+ and Zn2+.b) Tetrahedral Co2+ and Zn2+ are both diamagnetic.c) The ionic radius of Co2+ is significantly smaller than that of Zn2+.d) The visible spectra of complexes of Co2+ are similar to those of related complexes of Zn2+.

Ans:- a.* The spectral behaviour of Co2+ (d7, paramagnetic) is different from Zn2+ (d10, diamagnetic,colorless, no visible spectra). This difference is used in “in vitro” studies;* the ionic radii of Co2+ and Zn2+ are almost same;* Co2+ can tolerate similar coordination environments to Zn2+;* it is often possible to replace Zn2+ in a protein by Co2+ without greatly disturbing the proteinconformation.

41.8) Nature has chosen Zn(II) ion at the active site of many hydrolytic enzymes because(a) Zn (II) is a poor Lewis acid.(b) Zn (II) does not have chemically accessible redox states.(c) Zn (II) forms both four and higher coordination complexes(d) Zn (II) forms weak complexes with oxygen donor ligands.

Ans:- The unique features of zinc are* It exists only in +2 state. It has no chemically accessible redox states i.e., redox inactive andhence no redox side reactions.* There are no ligand field stabilization effects due to its d10 configuration. Hence there are noligand field constaints over the geometry. Its geometry and coordination number are onlydictated by ligands and the charge.* Usually zinc adopts usually tetrahedral geometry in biocomplexes (some square pyramidalcomplexes are also reported).* It is a good lewis acid next to copper.* It is a borderline hard acid and can bind to oxygen (Asp, Gu, H2O), nitrogen (His) andsulfur (Cys).

41.8) Dioxygen’s reduction potential is +0.816 mV at pH = 7 and hence is a good oxidizingagent. Yet it does not react with organic molecules under normal conditions. Explain.

Ans:- Dioxygen in the ground state exists in triplet state, whereas organic molecules are mostly insinglet state. Hence dioxygen is kinetically inert towards organics.

But it readily reacts with metals irreversibly. (That is why iron in hemoglobin is to beprotected in the hydrophobic environment of globin.)


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

7

41.9) What is Calmodulin (CaM)? Mention its role?Ans:- Calmodulin (CALcium MODULated proteIN) is a calcium binding protein present in all

eukaryotic cells. It can bind to and regulate a multitude of different protein targets, therebyaffecting many different cellular functions such as inflammation, metabolism, apoptosis,muscle contraction, intracellular movement, short-term and long-term memory, nerve growthand the immune response.

41.10) What are porphyrins? Draw the structures of different types of porphyrins in bio-logical systems.

Ans:- Porphyrins are the substituted porphins which are heterocyclic macrocyclics containing 4modified pyrrole rings interconnected at their alpha carbons via methine(=CH-) bridges.

The simplest porphin found in Hemoglobin is called porphyrin.The porphin in chlorophyll is called chlorin.In vitamin B12, the two of the pyrrole rings are directly connected to each other. This type

of porphin is called corrin.Porphyrin is a highly conjugated system and deeply colored. It is aromatic containing

26 -electrons, a Huckel number..

NNH

NHN

NNH

NHN

NN

NHN

Porphyrin Chlorin Corrine.g. Heme e.g. Chlorophyll e.g. Vitamin B12

Note: Actually above rings are substituted at various places by different substituents.

41.11) What are ionophores? Mention their role in biology.Ans:- An ionophore is a lipid soluble macrocyclic molecule, which transport ions (especially hard

cations) across the lipid bilayer of the cell membrane. Ionophores are involved in passivetransport.

There are two types of ionophores as follows;* Carrier ionophores which bind to a particular ion and facilitates its transport through the

lipid membrane. It shields the charge on ion from the surroundings and forms a lipophilic shellaround the ions.

* Channel forming ionophores, which form a hydrophilic pore (or channel) in a membrane,allowing ions to pass through it.

Ionophores are ‘charge and size’ selective. They usually coordinate through O and N.Chelation plays major role in stabilizing the complex. Selectivity depends on number ofcoordination bonds and confirmation of ionophore.

These are used to used to increase the permeability of biological membranes to certainions and also act as antibiotics.

Ionophores disrupt transmembrane ion concentration gradients, required for the properfunctioning and survival of microorganisms, and thus have antibiotic properties.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

8

Examples of ionophores with the ions upon which they act.Valinomycin (K+)Salinomycin (K+)Gramicidin A (H+, Na+, K+) ---- It is a transmembrane channel forming ionophore.Nonactin (NH4

+)Ionomycin (Ca2+)2,4-Dinitrophenol (H+)Monensin (H+, Na+) ----- used in cattle feed

* Crown ethers are the laboratory analogues of ionophores.

41.12) Why Gadolinium salts are used as MRI agents?* A good MRI agent should have following charactersitics:

1) High magnetic moment;2) Long electron-spin relaxation time;3). Low toxicity;

* Gd3+ salts like [Gd(dtpa)(H2O)]2-(gadopentetate dimeglumine) and [Gd(dota)(H2O)]-

(gadoterate meglumine) fit this for the purpose.

Practice questions1) Identify one significant role in biological processes for the elements Fe, Mo, Mn and Cu.

2) In biological systems, the metal ion involved in the dioxygen transport besides Fe isa) Co b) Zn c) Mg d) Cu

3) In photosynthesis, the predominant metal present in the reaction centre of photosystem IIis

(a) Zn (b) Cu (c) Mn (d) Fe

4) Zn in carbonic anhydrase is coordinated by three histidine and one water molecule. Thereaction of CO2 with this enzyme is an example of

(a) electrophilic addition (b) electron transfer(c) nucleophilic addition (d) electrophilic substitution.

5) The metals involved in nitrogenase are(a) Fe and Mg (b) Mo and K (c) Mo and Fe (d) Fe and K

6) The transition metal present in vitamin B12 is_______

7) The trivalent ion of lanthanoid element which is used as NMR contrasting agent is1) Gadolinium 2) Technetium 3) Cerium 4) Lutetium

8) Match the following1) Li+ A) Ulcer treatment2) Bi3+ B) Eczema3) Sb3+ C) Anemia4) Fe2+ D) Depression

9) The enzyme which removes C-terminal amino acid from a peptide is1) Carbonic anhydrase 2) Carboxy peptidase 3) Zymase 4) All


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

9

10) Which of the following metal is stored by metallothionein in biological systems?1) Fe 2) Cu 3) Zn 4) Co

11) The metals present in superoxide dismutase area) Zn & Fe 2) Cu & Fe 3) Mo & Cu 4) Cu & Zn

12) In human body, the most abundant nonmetal is _____ and most abundant metal is _____.

13) The copper containing non heme respiratory protein is1) Cytochrome-c 2) Hemerythrin 3) Hemocyanin 4) Myoglobin

14) The oxidation state of iron stored in ferritin is1) +2 2) +1 3) +3 4) 0

15) The enzyme which catalyzes the conversion of carbondioxide to bicarbonate is1) Carboxy peptidase 2) Superoxide dismutase3) Carboxy anhydrase 4) Hydrogenase

16) The number of methine bridges present in porphyrin is _______ and in corrin is _____ .

17) The ionophore used in the cattle feed to improve the permeability of Na+ and H+ ions is1) Crown ethers 2) Monesin 3) Ionomycin 4) Nonactin

18) Match the followingA) Silver sulfadiazine 1) Anti copper agentB) Selenium sulfide 2) Skin fungal infectionsC) tetrathiomolybdate 3) RadiodiagnosisD) Cardiolyte 4) Seborrheic dermatitis

19) The function of Na+ and K+ in biological systems is1) To maintain osmotic balance in the cells 2) Help in active & passive transport3) As charge carriers 4) All

20) Fluoride is present in the teeth in the form of1) CaF2 2) Na3AlF6 3) Ca5(PO4)3F 4) CaF2.CaCO3

21) Which of the following is a channel forming ionophore?1) Valinomycin 2) Salinomycin 3) Gramicidin 4) All

22) The porphin system in chlorophyll is called as1) porphyrin 2) chlorin 3) corrin 4) heme

23) The metal present in both superoxide dismutase and hemocyanin isa) Zn b) Fe c) Cu d) Ni

24) The metal ion present in urease is____ .

25) The ionophore valinomycin is highly selective for:a) K+ b) Na+ c) Mg2+ d) Ca2+


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

10

Hint: The stability constant of complex formed by valinomycin with K+ is 106, whereas thatwith Na+ is 10.

42) Non-heme iron-sulfur proteins are involved in:1. Electron transfer.2. Proton transfer.3. Both electron and proton transfer4. Oxygen transfer.

Explanation* Iron-sulfur proteins are proteins characterized by the presence of iron-sulfur clusters contain-ing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states.E.g., Ferredoxins, as well as NADH dehydrogenase, hydrogenases, nitrogenase etc.,* Iron-sulfur clusters are best known for their role in the oxidation-reduction reactions of mito-chondrial electron transport.* Additionally some Fe-S proteins regulate gene expression.* Fe-S proteins are vulnerable to attack by biogenic nitric oxide.In most iron-sulfur proteins, the clusters function as electron-transfer groups.

Additional information:1) Ferredoxins: are small proteins containing iron and sulfur atoms organized as iron-sulfurclusters. These biological "capacitors" can accept or discharge electrons, the effect being changein the oxidation states (+2 or +3) of the iron atoms. Thus ferredoxin acts as electron transferagents in biological redox reactions.

The following diagram illustrates the redox scheme between low-potential and high-poten-tial (HiPIP) ferredoxins containing Fe4S4clusters. The formal oxidation numbers of the iron ionscan be [2Fe3+, 2Fe2+] or [1Fe3+, 3Fe2+] in low-potential ferredoxins. The oxidation numbers ofthe iron ions in high-potential ferredoxins can be [3Fe3+, 1Fe2+] or [2Fe3+, 2Fe2+]

S

Fe

Fe

S

Fe

S

S

FeS

S

S

SS

Fe

Fe

S

Fe

S

S

FeS

S

S

SS

Fe

Fe

S

Fe

S

S

FeS

S

S

S

+e-

-e-

+e-

-e-

2-1- 3-

Following is Fe2S2 type of ferredoxin.

Cys

CysCysS

S

S

S

FeFe SS

Cys

Note: High potential iron-sulfur proteins (HiPIPs) form a unique family of Fe4S4 ferredoxinsthat function in anaerobic electron transport chains.

* Aconitase hydratase contains Fe4S4 cluster in active form and Fe3S4 cluster in inactive form.* Rubredoxin is considered as another iron-sulfur protein which does not contain inorganicsulfide. It is also an electron transport agent.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

11

Hemoglobin and Myoglobin* Hemoglobin(Hb) and myoglobin(Mb) are the heme containing metalloproteins (Note: ironporphyrin system is called heme). Both of them contain Fe(II) ion. Hemoglobin is present inRBC and helps in transport of dioxygen from lungs to tissues. Whereas, myoglobin storesdioxygen and is present in muscles.* Hb contains four heme units and four globular protein sub-units whereas Mb contains onlyone heme unit surrounded by a globular protein.

* Globin part prevents irreversible oxidation of Fe(II) ion by providing hydrophobic environ-ment. It enhances the selectivity for O2 binding. In hemoglobin, the tetramer allows forcooperativity which makes it more efficient in binding to dioxygen.* In deoxy-hemoglobin, four of the coordinated sites of iron are occupied by nitrogens ofporphyrin ring. The fifth site is occupied by Histidine residue (called proximal histidine) ofglobin. The sixth position is occupied by weakly bonded water molecule. Deoxy-hemoglobinis said to be in T-state (tense).

Another histidine group (distal), on the opposite side of the first histidine group, is situ-ated near the iron and assists the binding of dioxygen in ‘end on bent’ confirmation (Thisparticular bent confirmation discourages the binding of CO to heme iron. Without this bentrequirement, CO may have even more affinity with the iron. CO binds to hemoglobin 200Xstronger than dioxygen but binds 20,000X stronger with unprotected heme).


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

12

N

NH

N N

N N

CH3

CH3

CH3 CH3

O-OC-H2C-H2C CH2-CH2-COO

-

Fe2+

NH

N

proximal histidine

distal histidine

* Hb coordinated to dioxygen is called oxy-hemoglobin. It is also referred to as R-state(relaxed). In oxy-hemoglobin the sixth coordinated position of iron is occupied by dioxygenin ‘end on bent’ geometry.* In deoxy-Hb, the porphyring ring is dome shaped. Fe(II) is in high spin state and is para-magnetic. It is bigger in size (0.78 Ao) and situated above the plane of the porphyring ring.* In oxy-Hb, the iron is in low spin state and diamagnetic. It is smaller in size (0.61 Ao) andcan fit into the cavity of porphyrin ring (which now becomes planar). Now Fe(II) can moveinto the cavity of porphyrin ring and drags the proximal histidine which inturn triggers theconfirmational changes in other globin subunits and opening of other heme sites. As a result,enhances the binding capacity of other heme irons (cooperativity through allostery).

FeII

N

NH

N N

N N

CH3CH3

CH3 CH3O

-OC-H2C-H2C CH2-CH2-COO

-

N

NH

FeIII

Desoxy HbDomed porphyrin

High spin Fe(II) - out of ring

Oxy HbPlanar porphyrin

Fe(III) - still high spin - into the ringBent peroxy oxygen

OO

NH

N

H-bond

NH

N

N N

N N

CH3CH3

CH3 CH3

O-OC-H2C-H2C CH2-CH2-COO

-

* There are two theories to explain the nature of Fe in oxy-Hb or oxy-Mb.Pauling model: Suggests presence of low spin Fe(II) and singlet O2. Both are diamagnetic.Weiss model: Suggests presence of Fe(III) and superoxide radical anion(O2

-). Both areparamagnetic. But strong paramagnetic coupling results in diamagnetic nature. The O-O


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

13

stretching frequency is1105 cm-1 as obtained in resonance raman spectroscopy is consistentwith the fact that O2 is in superoxide form. This is more accurate and modern explanation.* Bisphosphoglycerate (BPG) reduces the affinity of Hb with oxygen. It binds at the centre ofcavity between two beta-Hb units and stabilizes the T-state. It is observed that at high alti-tudes (on mountains), the amount of BPG increases in the blood. This won’t affect the affinityof Hb with oxygen in lungs where the pO2 is very high but decreases the affinity in tissues andhelps in release of dioxygen by stabilising the T-state of Hb.* Bohr effect: The affinity of Hemoglobin with dioxygen decreases with decrease in the pH ofblood. Hence oxygen is released more efficiently in the tissues where CO2 concentration ishigh. The response of hemoglobin to changes in pH is called the Bohr effect.* The percent saturation with O2 curve of Hb against pO2 is sigmoidal whereas that of Mb ishyperbolic. This shows greater affinity of Mb with dioxygen even at low pO2 values.* Deoxyhemoglobin is the form of hemoglobin without the bound oxygen.The oxyhemoglo-bin has significantly lower absorption (660 nm) than deoxyhemoglobin (940 nm). This differ-ence is used for measurement of the amount of oxygen in patient's blood by pulse oximeter.

Mb - Hyperbolic curve

Hb - Sigmoidal curve

% s

atur

atio

n w

ith O

2

pO2

CYTOCHROMES* Cytochromes are, membrane-bound hemoproteins that contain 4 heme groups as co-factors and carry out electron transport. Most of them contain iron (either in Fe(II) orFe(III) oxidation states) at their active site.

They are found in the mitochondrial inner membrane and endoplasmic reticulum of eu-karyotes, in the chloroplasts of plants, in photosynthetic microorganisms, and in bacteria.* Cytochromes are classified based on heme type or heme iron coordination.

1) Cytochrome-a contains heme-a2) Cytochrome-b contains heme-b3) Cytochrome-c contains heme-c

* In cytochromes, the iron is hexacoordinated. The four coordination sites are occupied byfour nitrogens on pyrrole rings of heme group and remaining are occupied by usually Histi-dine, Cysteine residues.* Depending upon the ligand, the redox potential of a given cytochrome can be tailored tomeet specific need in electron transfer schemes.

The potentials are such that the electron flow is always fromcyt-b ----> cyt-c ----> cyt-a ----> O2

* In the mitochondrial electron-transfer chain, cytochrome-c accepts an electron from cyto-chrome-c1 and then transfersit to cytochrome c oxidase. Ultimately, the electron is used in thefour-electron reduction of O2* Only cytochrome-a has the ability to bind to O2 and reduce it. The CN- ion can bind stronglyto the sixth coordination site and stabilize Fe(III) in cytochrome-a. This makes cyt-a to stop


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

14

functioning in electron transfer reactions.

CYTOCHROME-C OXIDASE* Cytochrome-c oxidase (not cytochrome-c) is the major respiratory protein of animal andplant mitochondria. It catalyzes the oxidation of Fe(II) of cytochrome-c, and the reduction ofdioxygen to water by supplying four electrons. It contains two hemes (with two Fe3+) andthree copper atoms, arranged in three centers.

CYTOCHROME P450* Cytochrome P450 oxidases constitute a super family of monooxygenase cytochromes.* They are so named for the characteristic Soret peak at wavelengths near 450 nm whenthe heme iron is reduced (with sodium dithionite - Na2S2O4) and complexed to carbon mon-oxide.* These enzymes are primarily involved in steroidogenesis and detoxification.* They oxidise the alkanes to alcohols.* Their reaction with dioxygen involve higher oxidation states of iron, such as Fe(IV).* In all monooxygenases, only one oxygen atom in dioxygen is transferred to the substratewhile the other is converted into H2O.E.g.1) P450- +

3 2 3 2R CH + O + 2e + 2H R C-OH + H O

2)O

HH

O

HOH

P450

Camphor 5-Exo-hydroxy camphor

Mechanism:* The active site has Fe centre which is switched between II, III and IV oxidation states.* In step-1, RH replaces H2O while the low spin Fe(III) is converted to high spin Fe(III).* The substrate RH is bonded by hydrophobic interaction into the protein pocket close to theactive centre.* Only one oxygen atom is retained on the active site (step-5). Another is converted intowater.* Overall two electrons are utilized.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

15

NFe

3+N

NN

S(Cys)

OH H

NFe

3+N

NN

S(Cys)

NFe

2+N

NN

S(Cys)

OO

-

NFe

3+N

NN

S(Cys)

RH

RHe-

RH

RH

O2

e-O-

O-

NFe

3+N

NN

S(Cys)

RH

+2H+

-H2O

NFe

4+N

NN

S(Cys)

O

+H2O -ROH

RH

Compound-I with Fe(IV)

step-1 step-2

step-3

step-4step-5

step-6

low spin high spin

Additional questions:42.1) Give the structure of active site in Rubredoxin and mention its role?Ans:- Rubredoxin is a low molecular weight iron containing bacterial protein involved in electron

transfer. Sometimes rubredoxins are classified as iron-sulfur proteins. However, in contrast toiron-sulfur proteins, rubredoxins do not contain inorganic sulfide.

Rubredoxin’s active site contains an iron ion (either in II or III oxidation state) which iscoordinated by the sulfurs of four cysteine residues forming an almost regular tetrahedron.

Fe

S SS

S

Cys

CysCys

Cys

Active site of Rubredoxin

Rubredoxins perform one-electron transfer processes. The central iron atom changesbetween the +2 and +3 oxidation states. In both oxidation states, the metal remains high spin,which helps to minimize structural changes.

The oxidized state is reddish (due to a ligand metal charge transfer), while the reducedstate is colourless (because the electron transition has an energy of the infrared level, which isimperceptible for the human eye).

42.2) Ferredoxin (Fd) is a sulfur-containing protein which undergoes redox reactions in avariety of microorganisms; Fdox

+ + e- -------> Fdred , Eo = 0.439V at pH = 7.0. Will Fd

generate hydrogen gas from hydronium ions dissolved in water if the standard potential


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

16

for the reduction 2H+ + 2e- ---------> H2 is -0.421 at pH=7.0.Show the redox reaction and explain your answer.

Ans:- L: 2H+(aq) + 2e- -----------> H2(g) ; Eo = -0.421VR: Fdox

+ + e- -----------> Fdred ; Eo = 0.439V

Overall cell reactionFdox

+ + H2(g) + -----------> Fdred + 2H+(aq)Ecell = ER - EL = 0.439 - (-0.421)V = 0.86V ------- which is > 0

This means that rG < 0 (why? see the note). Hence reduction of Fdox+ is spontaneous and

NOT the liberation of hydrogen. Therefore H2 gas is consumed, and not produced in thisreaction.Note: 1) The electrode with low reduction potential is written on the Left hand side and thatwith high potential is written on the Right hand side of the galvanic cell. Left hand side halfcell is considered as anode and oxidation occurs in this cell. Whereas, Right hand side half cellis the cathode where reduction occurs.2) G = -nFE

42.3) In the following reaction ferredoxin-1 is the oxidised form of ferredoxin. State whetherthe following reaction is true or false?

nitrite reductase-2 3NO + ferredoxin-1 NH + ferredoxin-2

Ans:- False. This is a reduction reaction. Hence only reduced form of ferredoxin can reduce NO2-

to ammonia as follows.nitrite reductase-

2 3NO + ferredoxin-2 NH + ferredoxin-1 (reduced) (oxidised)

42.4) What prevents synthetic (simple) iron porphyrins from functioning as O2 carriers?Ans:- In the naturally occuring porphyrins, like Hemoglobin, there is a globular protein around

the heme groups. It prevents the irreversible oxidation of Fe(II) to Fe(III) and solvolysis ofFe(II)-O2 complex by providing hydrophobic environment around iron. It stops the formationof Fe-O2-Fe dimer.

However in synthetic porphyrins, protein part is absent. As a result, Fe(II) is oxidized toFe(III) and Fe-porphyrins easily dimerize to Fe-O2-Fe and then Fe-O-Fe (a -oxo dimer) andhence cannot function as oxygen carriers.

42.5) What is the active site in Rieske protein? Identify its role in bilological processes.Ans:- Rieske protein is an iron-sulfur protein (ISP) component of electron transfer chains like

1) cytochrome bc1 complex (found in mitochondria).2) the cytochrome b6f complex (found in photosynthetic systems).

It is a 2Fe-2S protein. It was first discovered and isolated by John S. Rieske.

The active centre is Fe2S2 cluster in which one iron is coordinated by two cysteine residuesand the other iron is coordinated by two histidine residues.

(Cys)S

(Cys)S

Fe

S

S

Fe

N(His)

N(His)

It is involved in electron transfer in respiration (in mitochondria) and photosynthesis (in


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

17

chloroplasts).

In mitochondria it accepts electron from Ubiquinol and transfers electron to heme iron incytochrome-c.

Whereas in Chloroplasts it accepts electron from Plastoquinol and transfers electron toheme iron in cytochrome-f.

42.6) Cytochrome C is a redox protein but myoglobin is an oxygen storage protein. Justifyin 2-3 sentences.

Ans:- In cytochrome C, the iron the six coordination sites are permanently occupied. It has fixedtwo axial ligands along with 4 pyrrole nitrogens from porphyrin ring. Hence it cannot store orcarry dioxygen. It is an electron carrier.

N

N N

Fe

N

(S)Cys

(N)His

Coordination pattern in Cytochrome C

But in Myoglobin or in Hemoglobin, the sixth coordinated site is occupied by losely boundwater which can be replaced by dioxygen easily. Hence Mb and Hb can store and carrydioxygen respectively. (sometimes the iron in deoxy hemoglobin is said to bepentacoordinated only i.e., sixth coordinated site is vacant)

N

N N

Fe

N

OH2

(N)His

N

N N

Fe

N

O

(N)His

O

Coordination pattern in Mb and Hb

42.7) Hemoglobin sometimes is called as dimer of dimers. Explain.Ans:- Hb contains two alpha and two beta subunits.

42.8) What are siderophores?Ans:- Siderophores are the multidentate anionic ligands released by microbes under extreme iron

deficiency conditions. Under highly oxidising conditions, iron is oxidized to Fe3+ and formsinsoluble Fe(OH)3. Hence not available to microbes. But microbes release siderophores toabsorb them. These molecules chelate with Fe3+ and make insoluble Fe3+ into soluble form andthus help in absorption of iron. Citrate is a simple siderophore.

Other e.g. Catecholates - like EnterobactinHydroxamate - like MycobactinDesferrichrome, Desferrioxamine B etc.,

Siderophores can fully satisfy the octahedral geometry requirements of iron withoutsignificant distortion and can bind flexibly.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

18

Note: 1) Methanobactin is a siderophore for absorbing Cu metal.2) In humans, iron is mobilized by transferrin.

42.8) What is the "Cooperative effect" in hemoglobin?Ans:- Coordination of one O2 leads to conformational changes in the protein chain leading to facili-

tated coordination of O2 by the other 3 sub-units in hemoglobin.

42.9) Explain why does carbon monoxide binds strongly to iron(II) porphyrin complexes butnot iron(III) porphyrins. Compare the infrared frequency of CO in this complex to thatof free CO.

Ans:- The Fe(II)-CO bond is strong because there are strong d -p * backbonding interactionswhich strengthen the Fe-C bond. This is much less important for Fe(III)-CO because of higherpositive charge on the metal.

The IR frequency of free CO is higher than that of CO in a porphyrin complex becausebackbonding adds electron density to the * antibonding orbital of the CO bond, decreasing itsstrength. (cf. Question 51)

42.10) Why are all the oxygen carriers that contain iron and porphyrins are found inside thecells?

Ans:- The inside cell environment is reducing and sustains Fe(II), whereas outside the cell the O2concentration is high and hence the probability of the oxidation of Fe(II) ions to Fe(III) in-creases.

42.11) Write the steps involved in the formation of hematin, a -oxo dimer, from free heme.

Ans:- Free heme in aqueous solutions is converted to hematin, a -oxo dimer when exposed todioxygen. The steps involved are summarized below.

Fe2+ + O2 Fe

2+O

O

Fe2+

OO Fe

2+ Fe3+

Fe3+

OO

Fe3+

Fe3+

OO

Fe4+

O Fe4+

O

Fe4+

O Fe2++ Fe

3+O

Fe3+

Binding of dioxygen molecule

Formation of peroxo complex

Formation of ferryl complexes in which iron is in +4 formal oxidation state

Hematin formation

+

Note: This -oxodiiron(III) moiety has a distinctive fingerprint that has made it easy toidentify this motif in proteins regardless of the geometry, type and number of ligands.

Magnetic susceptibility = 1.5 to 2.0 Bohr magnetons, indicating one unpaired electron. Itis due to strong antiferromagnetic coupling of iron centres through oxygen. (Actually thevalue should be more)

Asymmetric Fe-O stretching frequency at 730 - 880 cm-1


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

19

42.12) What is the active site in Hemerythrin? Mention its role in marine invertebrates?Ans:- Hemerythrin is a iron containing NON HEME oligomeric protein which transports O2 in

marine invertebrates. It is a respiratory protein.The monomer of hemerythrin is myohemerythrin, which is present in the muscles of

marine invertebrates and stores dioxygen. (hemerythrin contains 8 subunits)Deoxy forms are colorless, whereas oxy forms are violet pink in color.

Active site: The oxygen binding site is a binuclear iron centre. Deoxyhemerythrin containstwo high-spin ferrous ions bridged by hydroxyl group

The iron ions are coordinated to the protein through the carboxylate side chains of oneglutamate, one aspartate, and five histidine amino acid residues.

One iron is hexacoordinate and another is pentacoordinate.

Histidine

Histidine

Histidine

Histidine

N

N

NN

NN

N

N

N

N

OO

Fe2+

OFe

2+ H

OO

Asp

Asp

Histidine

The pentacoordinated Fe(II) binds to triplet dioxygen(3O2) and oxidized to Fe(III). Thenthe hydrogen on hydroxy group is transferred onto peroxide group. Now the second Fe(II) isalso oxidized to Fe(III). In this process, an oxo bridge is formed between iron ions.

Fe2+

Fe2+

O HO=O

O O

Fe3+

Fe3+

O

H

Deoxy formColorless

Oxy formViolet-pink

Unlike in hemoglobin, there is no cooperative effect observed in case of hemerythrin. Itsaffinity towards CO is less than with O2.

42.12) What is the active site in Hemocyanin? Mention its role?Ans:- Hemocyanin is respiratory protein containing two copper centres at the active site. It is a

dioxygen carrier suspended in the hemolymph (blood) of most molluscs and arthropods. Itcontains NO HEME.

The deoxy form contains Cu(I) ions and is colorless, whereas the oxy form containsCu(II) and is blue in color.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

20

In the deoxy form, each Cu(I) is coordinated to three histidine residues. Side-on bridgingcoordination occurs with dioxygen in its oxy form.

Histidine

Histidine

Histidine

NN

NN

NN

Cu+

Histidine

Histidine

Histidine

NN

NN

NN

Cu+

Histidine

Histidine

Histidine

Histidine

Histidine

Histidine

NN

NN

NN

Cu2+

ON

N

NN

NN

Cu2+

OO2

* Cu(II) has d9 configuration. The unpaired electrons in both Cu(II) ions couple by a bondinginteraction through the bridging peroxy ligand. Hence oxyHc is diamagnetic.Spectroscopic evidences of above oxyHemocyanin (oxyHc) structure;

1) Raman spectroscopy indicates symmetric binding and rules out mononuclear peroxocompound.

2) OxyHc is EPR silent indicating the absence of unpaired electrons.

42.13) How are the copper proteins classified? Describe their EPR behaviour?Ans:- In copper proteins, one or more copper ions are present as prosthetic groups. They are

broadly divided into three types as follows:

Type I :* Contain single copper atom, coordinated in distorted trigonal planar geometry to twohistidine and a cysteine residue. A loosely bound methionine residue is also present at axialdistant position.* These are called blue copper proteins or cupredoxins. They show beautiful intense bluecolor due to LMCT (Ligand to Metal Charge Transfer) at 600 nm. Charge transfer occursbetween Cys-S to Cu(II). There is transfer of electron density from non bonding orbitals ofsulfur atom of Cysteine to the empty d-orbitals. LMCT are Laporte permitted and henceshow intense absorption.* Small hyperfine EPR coupling constant (5x10-4 cm-1).* They have relatively high reduction potentials (> 250 mV)* Function as electron transfer agent.* E.g. Plastocyanin ( helps in electron transfer in PSII to PSI)

Type II:* Square planar geometry. Coordination through O or N. No coordination with S containingligands.* His, Tyr and H2O are the ligands. There is no cysteine.* There are non blue copper proteins. Only normal d-d transitions are possible. As they areLaporte forbidden, the color is not intense.* Normal hyperfine EPR coupling constant (18x10-4 cm-1) comparable to regular coppercoordination compounds.* Catalyse redox reactions.* E.g. Galactoseoxidase and superoxide dismutase


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

21

Type III:* Contain two copper centres, each of which are coordinated by three histidine residues.* Intense blue color in oxidised form due to LMCT from O2

- to Cu(II).* No EPR signal due to strong antiferromagnetic coupling between metal ions through thebridging ligand. The spins are paired up due to covalent overlapping of metal ions through thebridging ligand.* E.g. Hemocyanin, Tyrosinase.

More examples:*Ceruloplasmin, which helps in absorption of iron, is a Cu protein containing 7 Cu centresrepresenting types I, II and III.

It is involved in the process of oxidizing Fe(II) to Fe(III) during the transfer of iron fromtransferritin to ferrritin.

* Nitritereductase contains type II (substrate activation) and type I Cu centres (for e- transfer)

Others:e.g. CuA (binuclear copper centre) and CuB in cytochrome-c oxidase.

Cuz centre found in nitrous-oxide reductase. It has 4 copper centres coordinatedby 7 histidine residues and bridged by a sulfur atom.

41.14) Draw generally accepted structure of active site of the enzyme involved in nitrogenfixation. Indicate the site on which nitrogen first binds.

Ans:- Nitrogenase catalyses the reduction of nitrogen to ammonia.- +

2 3 2N + 8e + 8H +16MgATP NH + H 16MgADP + Pi All the nitrogenases consists of two subunits

i) M-cluster (FeMo cofactor) - containing Fe, S and Moii) P-cluster - containing Fe and S

According to modern view, the M-cluster is involved in the reduction of dinitrogen toammonia. The iron centres at the middle (shown in circles) are involved in binding ofdinitrogen. (Note: These irons are just having three bonds and with open configuration)

S

Fe

Fe

Fe

S

S

Fe

S

S

Fe

Fe

Mo

S

S

Fe

S

S(Cys)S O

N(His)

OO

M-cluster

The P-cluster contains cubane like [4Fe,4S] ferredoxins which are involved in the transferof electrons to M-cluster.

Note: The molybdenum, may be replaced by vanadium or iron in some organisms.

41.15) What is the action of Super Oxide Dismutase (SOD)?Ans:- Sequential additions of electrons to dioxygen during the action of oxygenase or oxidative

phosphorylation produce harmfull species like superoxide, hydrogen peroxide. These speciescan be eliminated by super oxide dismutases (SODs) and catalases.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

22

SOD converts superoxide ion into oxygen and hydrogen peroxide. Catalases then converthydrogen peroxide to oxygen and water.

O2- SOD

O2 + H2O2

H2O2 O2 + H2OCatalase

Usually SOD contains Cu-Zn active centre. The Cu and Zn atoms are connected throughan imidazole ring(of histidine).

N N

Cu

His

His

His

Fe

His

Asp

His

His

During catalysis, the Cu binds to superoxide and cycles between the +1 and +2 oxidationstates. Conversion of superoxide to oxygen occurs when the Cu is reduced from +2 to +1,and conversion of superoxide to hydrogen peroxide occurs when Cu is oxidized from +1 to+2.

SOD-Cu2+ + O2- SOD-Cu+ + O2

SOD-Cu+ + 2H+ + O2- SOD-Cu2+ + H2O2

Practice questions1) What is the role of globular protein in oxygen transport? cf. (42.4)

2) Which of the following is correct concerning the differences between hemoglobin andmyoglobin?

A) Hemoglobin is a monomeric protein and myoglobin is a tetrameric protein.B) Hemoglobin exhibits a hyperbolic O2 saturation curve while myoglobin exhibits a

sigmoid shaped curve.C) Hemoglobin exhibits cooperative binding of O2 while myoglobin does not.D) Hemoglobin exhibits a higher degree of O2 saturation at all physiologically relevant

partial pressures of O2 than does myoglobin.

3) The metal present in carbonic anhydrase is(a) Cobalt (b) Nickel (c) Zinc (d) Magnesium.

4) In biological systems, the metal ions involved in electron transport are(a) Na+ and K+ (b) Zn2+ and Mg2+

(c) Ca2+ and Mg2+ (c) Cu2+ and Fe3+

5) Which one of the following statements for hemoglobin is NOT correct?(A) The binding with O2 is weaker in comparison with myoglobin.(B) Iron is 5-coordinated.(C) Iron is coplanar with the porphyrin ring in the absence of oxygen.(D) The oxidation state of iron is +2.

6) When a reduced cytochrome transfers an electron from its Fe (II) to the bound O2,


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

23

(a) The bond order of O2 is reduced by one and 2O decreases.

(b) A metal bound superoxide is formed and 2O decreases.

(c) A metal bound superoxide is formed and 2O increases

(d) The bond order of O2 is reduced by one and 2O increases

7) Match the followingI II

P) Liver alcohol dehydrogenase 1) Cu at the active siteQ) cytochrome C oxidase 2) Fe and Cu at the active siteR) Hemocyanin 3) Zn at the active siteS) Myoglobin 4) Fe at the active site

5) Mo at the active site6) Cu and Zn at the active site

(a) P-6, Q-2 R-1, S-4 (b) P- 3, Q-2, R-1, S-4(c) P-3, Q-2, R-4, S-5 (d) P-5, Q -6, R-1,S-2

8) Match the followingI II

P) Cytochrome C I) ZincQ) Calmodulin II) PotassiumR) Chlorophyll III) MagnesiumS) Alcohol dehydrogenase IV) Calcium

(a) P - I Q - II R -III S - IV(b) P - II Q - III R -IV S - I(c) P - III Q- IV R - I S- II(d) P-V Q- IV R - III S - I

9) Which statement is the correct description of hemerythrin?1) A heme protein with one Fe centre at the active site..2) A metalloprotein containing two Fe centres at the active site.3) A non-heme protein with one Fe centre at the active site.4) A heme protein without Fe centre at the active site.

10) Match the following:P. Ferritin I. Electron transportQ. Vitamin B12 II. IonophoreR. Cytochromes III. Oxygen transportS. Valinomycin IV. nitrogen fixation

V. Organometallic enzymeVI. Iron storage.

(a) P - VI Q - IV R-II S - I(b) P - I Q - III R-VI S - IV(c) P - III Q- V R - IV S- VI(d) P-VI Q-V R - I S - II

Note: Vitamin B12 is an organometallic as there is a Co-C bond (metal-carbon bond). In the


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

24

native form there is a methyl or methylene group attached to Cobalt. Whereas it is attached toa -CN group during isolation. Hence the name cyanocobalamine.

11) The iron containing respiratory protein without heme is _____.

12) The Cu-siderophore used in absorption of Cu by bacteria is1) Methanobactin 2) Enterobactin 3) Mycobactin 4) Desferrioxamine

13) Inactive form of Aconitase hydratase, which catalyzes the stereospecific isomerization ofcitrate to isocitrate via cis-aconitase, contains

1) Fe4S4 cluster 2) Fe2S2 cluster 3) Fe3S4 cluster 4) Fe(S-Cys)4 cluster

14) Choose the incorrect statement about Types 1, 2 and 3 centres in copper proteins?a) Type 1 centre exhibits an intense LMCT band in the electronic spectrum.b) Ceruloplasmin contains all Types of copper centres.c) Type 3 centre contains two Cu centres which are antiferromagnetically coupled.d) Plastocyanin contains a Type 1 Cu centre.e) Type 2 centre does not give rise to an EPR signal.f) Hemocyanin contains Type 2 Cu centre.

15) Iron-sulfur proteins where one Fe atom is coordinated by two His residues are named:A) Poison iron-sulfur proteinsB) Rieske iron-sulfur proteinsC) Warburg iron-sulfur proteinsD) Homo iron-sulfur proteinsE) Cytochrome C iron-sulfur proteins

16) Which statement correctly describes the function of cytochromes P-450?1) Cytochrome P-450 acts as monooxygenases and catalyses the insertion of O into a C–H

bond.2) Cytochrome P-450 couples to cytochrome-c in the mitochondrial electron-transfer

chain.3) Cytochrome P-450 act as dioxygenases.4) Cytochrome P-450 contains high-spin Fe(III); this directly binds O2 and acts as an O2

carrier.

17) Which statement about Fe–S proteins is INCORRECT?1) A rubredoxin contains an FeS4 unit, each S coming from a Cysteine residue.2) A [2Fe–2S] ferredoxin contains six S donors, two of which are S2– ligands.3) A [4Fe–4S] ferredoxin contains a cubane core.4) In a [4Fe–4S] ferredoxin, four redox couples that make use of the four Fe centres are

accessible in Nature.

Note: In [2Fe-2S] iron-sulfur cluster, the 2 iron atoms are complexed to 2 inorganic sulfidesand 4 sulfur atoms of cysteines from the protein.The incorrect statement is the 4th one.

18) Nitrogenase contains:A) an Fe-protein and an FeMo-protein that operate in conjunction with each other.B) an FeMo-protein in which the active site consists of a single [3FeMo-4S] cluster.


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

25

C) an FeMo-protein in which the P-cluster is in an irreversibly reduced state.D) an Fe-only containing protein in which the P-cluster is the active site.

19) Enterobactin is used by microbes to chelate and absorb1) soluble Fe(II) 2) insoluble Fe(III) 3) soluble Fe(III) 4) insoluble Fe(II)

20) The iron-sulfur protein which does not contain inorganic sulfides is________ .

21) The enzyme which contains cubane like ferredoxin is1) Carbonic anhydrase 2) Urease 3) Zymase 4) Nitrogenase

22) Which of the following statement is not correct?a) The sixth coordination position in T-state of hemoglobin is occupied by losely bound

water molecule.b) The affinity of Hemoglobin with CO will be even more without the presence of distal

histidine residue.c) In T-state of hemoglobin, the iron is in low spin and has bigger size (0.78 Ao).d) In R-state of hemoglobin, the iron is in low spin state and has smaller size (0.61 Ao)e) The porphyrin ring in T-state of hemoglobin is dome shaped.

23) Biphosphoglycerate stabilizes the T-state of hemoglobin. As a consequence:1) Affinity of hemoglobin with dioxygen improves in lungs.2) Affinity of hemoglobin with dioxygen decreases in lungs.3) More dioxygen is released in the tissues where the pO2 is less.4) Less dioxygen is released in the tissues where the pO2 is high.

24) The type of cytochrome with lower reduction potential is1) Cytochrome a 2) Cytochrome b3) Cytochrome b 4) All with same reduction potentials

25) In cytochrome-c oxidase, the number of iron centres is _____ and the numbe of coppercentres is ______ .

26) The metal found at the active site of Rieske protein isa) Ni b) Fe c) Cu d) Zn

27) The 2Fe-2S protein found in mitochondria and in photosynthetic systems which involve intranfer of electrons is

a) Ferredoxin b) Rubredoxin c) Rieske protein d) All

28) The respiratory non heme iron protein containing binuclear iron centre isa) Hemerythrin b) Hemocyanin c) Cytochrome-c d) Hemoglobin

29) Choose the correct statement about copper proteins.a) EPR sigal is abnormal in Type-II copper proteins.b) Type-III copper proteins are intense blue in color due to MLCT.c) Hemocyanin is a Type-II copper protein.d) Plastocyanin is a Type-I copper protein.

30) Type I and III copper proteins are intense blue in color due toa) LMCT b) d-d transitions c) MLCT d) f-f transitions


V.ADITY

A VARDHAN

HTTP://W

WW.A

DICHEMADI.C

OM

26

31) EPR signal is absent for which of the following blue copper protein:a) Superoxide dismutase b) Hemocyaninc) Plastocyanin d) Nitrite reductase

32) The number of MgATP’s required in the reduction of N2 to ammonia isa) 4 b) 12 c) 16 d) 6

33) The blue copper protein which helps in the transport of iron isa) Ceruloplasmin b) Plastocyanin c) Tyrosinase d) Nitritereductase

34) The part of nitrogenase enzyme which is involved in transfer of electrons isa) M-cluster b) Molybdenum c) P-cluster d) protein part

35) During the oxidations catalysed by cytochrome P450, one atom of dioxygen enters intothe organic substrate and the other atom of oxygen finds its way into

a) CO2 b) H2O2 c) H2O d) CO

36) Iron sulfur clusters in biological systems are involved ina) proton transfer b) atom transferc) group transfer d) electron transfer

Csir Qn Papers

Documents

Transcript of Csir Qn Papers